A unique feature worth mentioning has to be the Quad Coil springs. Riding the Skorpion Skates on rough surfaces is not at all fatiguing without the vibrations.
What I liked about the Multi-terrain version is the stability due to larger footprint. It's harder to fall on the Multi-terrain, making it more friendly to beginners.
What I don't like about the Multi-terrain version is the rolling resistance of the wheels: The Quadline version is lightning fast compared to the Multi-terrain version on most if not all surfaces.
The bottom line: I've tried inline skates before and I liked my Skorpion skates more. If you are a beginner or intend to do Nordic skating (or kite skating), Skorpion Multi-terrain is for you; else, the Quadline version is highly recommended.
Conversion kits are available just in case you want to convert the Multi-terrains to Quadlines. (But Quadlines can not be *officially* converted to Multi-terrains)